There are no restrictions on the contents of a Phar stub, except for the requirement
that it conclude with __HALT_COMPILER();. The closing PHP tag

?>

may be included or omitted, but there can be
no more than 1 space between the ; and the close tag

?>

or the phar extension will be unable
to process the Phar archive's manifest.

In a tar or zip-based phar archive, the stub is stored in the
.phar/stub.php file. The default stub for phar-based
Phar archives contains approximately 7k of code to extract the contents
of the phar and execute them. See Phar::createDefaultStub()
for more detail.

The phar alias is stored in a tar or zip-based phar archive in the
.phar/alias.txt file as plain text.